The present invention relates to an FSK (Frequency Shift Keying) receiver and, more particularly, to an FSK receiver in which a wave frequency-modulated with a binary digital signal by an FSK modulating scheme is received by a quadrature detecting scheme.
In recent years, a compact receiver has been developed in accordance with the progress of an integrated circuit technique. However, in the field of radio communications, since the basic circuit design has not changed, a decrease in size of the receiver almost reaches its limitation due to the presence of elements which cannot be integrated or which cannot easily be integrated. For example, in a superheterodyne receiver, a high-frequency filter, an intermediate frequency filter, and the like occupy a large area.
By entirely changing the basic circuit design, a circuit system without a high-frequency filter, an intermediate frequency filter, and the like which occupy a large area has been proposed. For example, a direct conversion scheme in an FSK receiver is known. This direct conversion scheme, as described in, e.g., U.S. Pat. No. 4,521,892, a channel frequency is set equal to a local oscillation frequency to generate a beat component between the frequency of a received signal and the local oscillation frequency, and only a baseband signal is extracted by a low-pass filter. The beat component is amplitude-limited by a limiter and then demodulated.
The direct conversion scheme has the following characteristic feature. Since the channel frequency is equal to the local oscillation frequency, an intermediate frequency becomes zero and no image frequency is present. This means that any filter, having excellent selectivity, for attenuating an image frequency is not required in a high-frequency amplifier and an intermediate frequency amplifier. Therefore, a compact, lightweight FSK receiver using the direct conversion scheme can be obtained.
A channel filter for attenuating an adjacent channel interference wave can be constituted by a low-frequency active filter because the intermediate frequency of the channel filter is zero. This channel filter can advantageously be realized in integrated circuit.
As described above, a high-frequency filter, an intermediate frequency filter, and the like are not required by employing the direct conversion scheme, and a compact, lightweight FSK receiver can be obtained. However, a conventional direct conversion scheme uses quadrature baseband signals, and a mixer circuit which must have a single-balance type circuit arrangement.
This single-balance type circuit is easily saturated when a field intensity is increased. In particular, when a power supply voltage is low, this saturation is conspicuous. When the saturation occurs, intermodulation easily occurs to degrade circuit characteristics. A market in the field of communications has been active in recent years, and the frequency of use of radio waves is increased, so that a field intensity is locally increased. As a result, intermodulation easily occurs.